1. Field of the Invention
The present invention relates generally to adjustable devices that provide selectable levels of dampening and/or stiffness, and more particularly, but not by way of limitation, to an improved vibration mitigation assembly for suppressing vibrations of civil structures, such as bridges.
2. Description of the Related Art
A recent report (1995) by the Federal Highway Administration indicates that approximately 23% of the bridge inventory in the U.S. is currently rated as deficient. Ten percent of all bridges in the U.S. are considered structurally deficient, and that percentage is growing each year. If those bridges are not repaired or replaced, then they will have to be derated or closed. With over 550,000 bridges in use, it is easy to appreciate the enormous negative impact that a decaying bridge infrastructure will have on the nation's economy.
Projections made by FHWA to Congress on the cost to repair the bridge infrastructure are now estimated to exceed $6 billion per year over a 25-year period. That will require a ten-fold increase in the present level of funds appropriated each year in the federal and state transportation budgets for bridges. The U.S. congress, state legislators, and the taxpayers continue to balk at the need to increase highway maintenance revenues. However, bridges along the interstate system are particularly critical to the continued growth of commerce.
The interstate roadway system was constructed over a span of five decades. The design of bridges constructed during this period mirrored the limitations of available capital and inaccurate projections on the growth of trucking commerce. As a result, many interstate bridges are now near the end of their useful service life. The simultaneous repair or replacement of all those thousands of obsolete bridges over a short period of time is not economically feasible. Departments of transportation around the country are now anxious to find an alternative means of dealing with the decay of the bridge infrastructure.
Suspicion among the public is that over-the-road trucks are to blame for the accelerated decay of America's bridges. The growth of trucking commerce in the U.S. has far exceeded the expectations of transportation economists. That growth reflects the extremely robust economy of the nation since the end of World War II. The trucking industry would argue that as daily truck counts increase so do the collected fees levied on trucks, which are intended to offset the cost increment in roadway maintenance that results from truck traffic. In addition, the federal government's increase in maximum permissible weight of a truck on the interstate system from 72,000 lbs. to 80,000 lbs. in the early 1980's is also considered by many to have had a significant negative impact on the useful life remaining of a bridge. The trucking industry, however, was assessed additional user fees to compensate for increased wear caused by heavier loads.
While increases in truck counts and truck weights are important, it is becoming increasingly clear that the useful life of bridges is now most affected by the deleterious vibratory characteristic of a heavy truck chassis. That is, it has been found that the low frequency sprung mass modes of heavy trucks correspond very closely to several of the fundamental modes of a majority of bridges. A review of the literature and surveys of truck manufacturers indicates that all heavy trucks, whether outfitted with leaf spring suspensions, air suspensions and/or passive hydraulic dampers, possess vibration modes between 1.5 Hz. and 5 Hz. What is disconcerting is that a survey of continuous plate girder bridges with spans ranging from 60 feet to 150 feet (which represents approximately 50% of the highway bridges) reveals that the generic design common to interstate bridges almost always results in bridge vibration modes that coincide with the suspension vibration modes of the truck. This is true whether the bridge is constructed of steel or precast concrete.
Thus, the core of the problem is that a vibrating truck chassis oscillates at a frequency that resonates near one or more of the bridge fundamental modes. That resonance condition results in dynamic loads imposed on the bridge structure that are typically 1.5 to 2.5 times the weight of the vehicle measured statically. The bridge fatigue design code that is relied on by bridge engineers to assess the remaining life of bridges suggests that a dynamic magnification factor of 1.15 is realistic, with a maximum magnification factor of 1.3 recommended for only the most conservative analysis.
Bridge researchers that have been investigating bridge/truck vibration resonance are now proposing that fatigue life prediction codes be revised to reflect the much larger dynamic impacts that occur if a bridge's modal frequencies correspond to the vibration suspension modes typical to heavy over-the-road trucks. Those revised codes will inevitably predict significantly less remaining service life for bridges now in use.
Correcting the fatigue codes does not alleviate the immediate problem, but it does provide a more reliable guideline for the design of future highway bridges. The highway engineering community is now considering an alternative approach, bridge friendly truck suspensions. However, while new trucks can be made bridge-friendly by design, trucks presently on the road will continue to pose a serious problem to bridge integrity.
To this end, what is needed is a low-cost means of fitting truck suspensions with technology that can automatically change the truck suspension vibration characteristics to avoid resonating with a bridge while the truck is passing over the bridge. It is to such a vibration mitigation assembly that the present invention is directed.